The apparatus of the present invention relates to the deflection circuit art for a CRT display apparatus. Specifically, a vertical drive circuit for providing current to a vertical deflection yoke is described.
Cathode ray tubes (CRT's) appear in many display devices known in the art. In order to deflect a beam produced by a CRT, it is necessary to apply either an electric or a magnetic field to the CRT in both the horizontal and vertical axes. In the television receiver art, a magnetic field is used to move the electron beam from the top of the screen to the bottom. The magnetic field is generated by a vertical deflection yoke known to those skilled in the art.
The vertical deflection yoke requires a current for positioning the electron beam along the vertical axis. A linearly changing current produces a change in the position of the beam. In the television art, the current is initially established so that the beam is at the top of the screen. The current is thereafter changed, forcing the beam to move towards the bottom of the CRT. During this time, referred to as the "trace" period, an image is formed on the CRT.
At the conclusion of the trace period, the current through the deflection yoke is rapidly changed to return the electron beam to the top of the picture tube. During this retrace period, the beam is shut off or "blanked" so that no image is formed during retrace.
The power dissipation in the drive circuitry associated with the yoke has been regarded as excessive by those skilled in the art. In the past, power efficiency has been improved by using a lower source of voltage for driving current through the yoke during the trace period and using a much higher voltage for driving current through the yoke during retrace.
To carry out this technique, a separate power supply has been used in the past to supply a much higher voltage to the yoke during retrace. By using two separate power supplies, voltage on drive circuitry may be kept lower during trace, thereby keeping power consumption to a minimum. The additional power supply, however, adds weight and cost.